Conventionally, there is a radio communication system which transmit a data by using an impulse. In the radio communication system using the impulse, it is possible to achieve a transmission unit by, for example, a pulse generator and transmission amplifier. Consequently, in the radio communication system, it is possible to achieve a reduction in the number of components, in costs and so on, compared to a radio communication system such as a portable telephone and so on, since an oscillator, mixer and the like are not used.
In the radio communication system using the impulse, for example, transmission of data of large volumes not less than 1 Gbps is investigated, and use of a frequency band known as a millimeter wave band, such as 70 GHz to 100 GHz, is also investigated. The millimeter wave band suffers little radio wave attenuation due to absorption by the air, compared to other frequency band, therefore in the radio communication system using the impulse, it is possible to transmit a radio signal over several kilometers or more.
Next, a radio communication by an impulse system is described using FIG. 13A to FIG. 15B. Of these drawings, FIG. 13A illustrates a configuration example of a radio communication apparatus which uses impulses. The radio communication apparatus 200 includes a pulse generator 201, band pass filter (BPF) 202, transmission amplifier 203, switch 204, antenna 205, reception amplifier 206, BPF 207 and wave detector 208.
The transmission data is input to the pulse generator 201 and converted into a pulse (or impulse) corresponding to the transmission data. FIG. 14A and FIG. 14B illustrate examples of the correspondences between transmission data and pulse.
Returning to FIG. 13A, the pulse output from the pulse generator 201 is input to the BPF 202, and only an energy element of the used frequency band is extracted. FIG. 13B illustrates an example of a transmission band of the BPF 202. As illustrated in FIG. 13A, a millimeter wave pulse (or wave bundle) which oscillates near central frequency of the transmission band is output from the BPF 202.
Thereupon, the millimeter wave pulse is transmitted into space from the antenna 205 via the transmission amplifier 203 and switch 204, as a radio signal. For example, in radio communication based on a simplest impulse method, data transmission is performed by ON/OFF modulation in which one bit indicates the presence or absence of the millimeter wave pulse. FIG. 14C illustrates an example of the radio signal which is transmitted from the radio communication apparatus 200.
Returning to FIG. 13A, the radio communication apparatus 200 is able to receive the radio signal which is transmitted from another radio communication apparatus, and can receive the radio signal via the antenna 205, switch 204, and reception amplifier 206. In the radio signal (millimeter wave pulse) amplified by the reception amplifier 206, only energy element of the transmission band is extracted by the BPF 207. The wave detector 208 can detect the output of the BPF 207 (for example, by envelope wave detection) and can output the reception data.
FIG. 15A and FIG. 15B are graphs each representing an example of a spectral distribution of the radio signal transmitted from the radio communication apparatus 200, for example. These graphs can be obtained by converting the radio signal on a time axis to the radio signal on a frequency axis by applying a fast Fourier transform (FFT) to the radio signal transmitted from the radio transmission apparatus 200 (or the output signal of the transmission amplifier 203).
FIG. 15B illustrates an enlarged graph of the spectral distribution in the frequency band from 70 GHz to 100 GHz, from the graph in FIG. 15A. In these diagrams, the horizontal axis represents the frequency and the vertical axis represents signal intensity.
As illustrated in FIG. 15A and FIG. 15B, for example, there is a portion where peak of signal intensity is higher than the other frequencies. The portion where the signal intensity peak is higher than the frequencies may be called a bright line section or bright line spectrum, for example. The bright line spectrum is generated, for example, because in the millimeter wave pulse such as those illustrated in FIG. 14C, the energy of the pulses is concentrated at a prescribed frequency (for example, 80 GHz or 90 GHz) and a unipolar return to zero (RZ) type pulse generator 201 is used.
On the other hand, in various radio communication apparatuses, and not just the impulse system, the signal intensity for a per unit of frequency is stipulated by ordinance or specification, or the like, and the radio communication apparatus is designed and manufactured, and the like, so as to come within the stipulated range.    Patent Document 1: Japanese Laid-open Patent Publication No. 2008-205733    Patent Document 2: Japanese Laid-open Patent Publication No. 2010-98481    Non-Patent Document 1: ETSI EN 302 217-3 VI.3.1 (2009-7)
However, there is a case where the peak of the bright line spectrum described above is higher than a threshold value. Normally, a circuitry included in the radio communication apparatus and the like, is designed or manufactured in accordance with the peak of the signal intensity; for example, the transmission amplifier 203 of the radio communication apparatus 200 illustrated in FIG. 13A, or the like, is designed and manufactured in accordance with the peak signal intensity in the bright line spectrum.
Consequently, if the bright line spectrum having the peak higher than the threshold value is generated, then the circuitry, and the like, included in the radio communication apparatus 200, such as the transmission amplifier 203, becomes more expensive and the consumed power becomes larger, compared to a case where the peak is equal to or lower than the threshold value.
Conversely, if the peak of the bright line spectrum can be made lower than the threshold value, then it is possible to transmit the radio signal with low transmission power, compared to the case where the bright line spectrum is higher than the threshold value. If the radio signal can be transmitted with low transmission power, then it is possible to transmit the radio signal over a distance at or exceeding a threshold value, by making the transmission power higher than hitherto.
Furthermore, for example, if an upper limit value for the signal intensity per unit frequency is stipulated by ordinances or specifications, etc., then it is possible to observe the ordinances or specifications, etc., of this kind by designing and manufacturing the transmission amplifier 203, or the like, in such a manner that the signal intensity peak of the bright line spectrum is equal to or lower than the upper limit value.